1. Field
The present invention relates to microelectromechanical devices and especially to a microelectromechanical structure and a microelectromechanical device, as defined in the preambles of the independent claims.
2. Description of the Related Art
Microelectromechanical systems, or MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. MEMS structures can be applied to quickly and accurately detect very small changes in physical properties.
Motion limiters are commonly used in microelectromechanical structures to control the maximum distance that a movable structure inside a device can displace. Another purpose for motion limiters is to dissipate energy in a controlled way in case a movable part of a MEMS structure collides with an anchored part due to a sudden high acceleration event. Such an event may occur, for example, when a device is accidentally dropped to the floor on a manufacturing line. Design distances between parts are also approached during element testing.
In collision, the primary purpose of a motion limiter is to protect the device from breakage, often by preventing fragile points of structures, e.g., sharp corners or narrow beams touching other surfaces. However, the motion limiter itself may also break if it is not robust enough. The robustness of a motion limiter can be enhanced, for example, by applying a hard coating over the structure, or by increasing the contact area between the contacting surfaces. The use of coatings is restricted to a limited number of cases only and cannot be conveniently applied when the contacting bodies are designed to move laterally within a single structural layer. On the other hand, increasing the contact area between the surfaces increases the risk of stiction between the contacting surfaces.
Traditionally a lateral motion limiter consists of a surface, which is locally raised from its environment. The contact surface of such a design has traditionally been flat or convex in shape.
Design with a flat contact surface gives a good protection against breakages when the relative movement of the contacting bodies is normal to the surface of the motion limiter and the contacting bodies are not laterally tilted. However, in case the bodies contact in an angle, the area of the contact surface is drastically reduced and the risk of breakage of the motion limiter is increased.
Use of convex shape motion limiters reduces the dependence of the contact area on the contact angle. However, the obtainable contact area becomes relatively small, unless the radius of the curvature of the limiter surface is made very large, which is unpractical in most of the cases.